Surface acoustic waves containing compressional and shear components in phase quadrature propagate non-dispersively along solid surfaces. This phenomena was predicted by Lord Rayleigh in the 1880's. More recently, microminiature SAW devices have been fabricated in which the surface acoustic waves are generated by electrical signals which are converted to acoustic signals by transducers formed on piezoelectric materials. These devices are used for the analog processing of electrical signals.
Significant SAW devices include bandpass filters, resonators and oscillators and pulse compression filters. System applications for such devices are numerous and include color television, radar, sonar, communication, non-destructive testing and fast Fourier transform processors.
The basic SAW structure comprises an InterDigitated metal film Transducer (IDT) deposited on a planar optically polished surface of a piezoelectric substrate, such as lithium niobate or quartz niobate. In Reflective Array Compressor (RAC) SAW devices the acoustic wave generated by the IDT is propagated along an array of suitably angled reflective slots etched into the substrate surface. These slots form a dispersive delay line grating. The spacing of the slots determines the frequency selectivity of the grating and the depth of the slots determines the amplitude weighting applied to the input pulse. Two such gratings are arrayed side-by-side on the substrate surface. The slots on the second grating are matched to the first to reform and counter-propagate the SAW beam parallel to but laterally displaced from the incident SAW beam (McGraw-Hill Encyclopedia of Electronics and Computers, 1984, pp. 793-796).
Slot depths are typically 1/100th of the acoustic wavelength, and slot spacings are typically one wavelength between centers. Needless to say, such stringent requirements necessitate extremely precise fabrication techniques. In practice, there is a significant amount of device-to-device variation, due to the sensitivity of device performance to fabrication steps.
In addition, even if fabrication technology were perfect, it is difficult to exactly predict device performance from device design because of substrate variability.
Due to these difficulties, viz., imperfect device design, imperfect device fabrication and substrate variability it is important to develop an ability to correct device performance after fabrication.
Phase compensation of pulse compression SAW gratings has been suggested as a means for improving device response by Williamson et al. in L-Band Reflective-Array Compressor with a Compression Ratio of 5120, Ultrasonics Symposium Proceedings, Williamson et al., IEEE, New York, 1973 pp. 490-493. Williamson et al. contemplate placing a metal film of variable width between the two grating structures to slow the wave and advance its phase (p. 492). In practice, the photolithographic processing required to define the film pattern perturbs both the amplitude and phase response of the wave and precludes independent phase adjustment beyond 2 degrees r.m.s. or less.
No known amplitude-compensation technique, short of complete iteration of device fabrication with a modified groove-depth profile, has previously been demonstrated.